1. Field of the Invention
This invention relates to devices for measuring strain in the polish rod of a rod-pumped well unit, thus giving an indication of the load on the rod string, and more particularly relates to devices which are removably attachable directly to the polish rod of a well unit for monitoring and/or controlling the operation of the well unit.
In rod-pumped well units of the type commonly used to produce oil from a subterranean formation, which units conventionally include a downhole pump attached to a reciprocal rod string extending to the surface, and which include a prime mover for driving a walking beam or the like, the rod string normally includes an upper section designated the "polish rod", which is connected to the walking beam by a clamp and bridle arrangement, all as is well known. In the operation of such well units, it is frequently desirable to utilize means to monitor and/or control the operation of the unit to maximize the efficiency thereof and to prevent equipment damage in the event of equipment malfunction or of reaching a "pumped-off" condition in the well.
2. Description of the Prior Art
The control of rod-pumped well units, prior to this invention, has been accomplished utilizing transducers positioned between a hangar bar and a clamp on the polish rod. These transducers may be strain gages bonded to compression blocks, exemplified by U.S. Pat. Nos. 3,343,409; 3,355,938; 3,457,781 and 3,527,094, or hydraulic transmitters as exemplified by U.S. Pat. Nos. 3,306,210 and 3,359,791. Alternatively, control of rod-pumped wells has heretofore been accomplished utilizing motor current transducers as exemplified by U.S. Pat. Nos. 3,440,512 and 3,610,779.
More recently, control of rod-pumped well units has been accomplished utilizing a strain gage transducer welded to the top flange of the walking beam of the unit.
All of the above prior art systems have been successful to varying degrees, but they are all subject to certain disadvantages and inconveniences. For example, the last-mentioned system using a strain gage transducer welded to the walking beam is a permanent installation which ties up the transducer substantially permanently. The motor current transducers give an indirect indication of actual conditions in the rod string, since they are not directly connected thereto. The transducers positioned between the hangar bar and the polish rod clamp, whether strain gages or hydraulic transmitters, require considerable mechanical effort for installation, including removing the weight of the rod string from the hangar bar. Additionally, these transducers positioned between the hangar bar and the polish rod clam in effect "weigh" the rod string, rather than measuring actual strain in the rod string. It has been found that a direct measurement of strain in the rod string is preferable to the indirect measurements obtained by prior art systems of controlling well units. Further, the direct measurement devices of this invention are much more convenient to install than prior art devices and can be removed and used on different wells with a minimum of effort.
Thus, it is apparent that there has long been a need for a measuring device for rod-pumped well units which is simple, easy to install and remove, and which provides a direct measurement of the strain in the rod string.